1. Introduction
This invention relates to the purification of phenolic resins having entrapped acid residues. In a preferred embodiment, this invention relates to the purification of phenolic resins having phenolic hydroxyl groups condensed with an acid halide. The purification process comprises treatment with a mixed acid and base ion exchange resin to remove acid salt contaminants.
2. Description of the Prior Art
Phenolic resins such as novolak resins and polyvinyl phenol resins are known and used as binders for many coating compositions. A major use of such resins is for the formulation of photoresist compositions. Novolak resins are disclosed as photoresist binders generally in U.S. Pat. No. 4,404,272. Polyvinyl phenol resins are disclosed as photoresist binders in U.S. Pat. No. 3,869,292.
The formation of phenolic novolak resins by condensation of a phenol with an aldehyde is well known in the art and described in numerous publications including the Kirk Othmer Encyclopedia of Chemical Technology, Volume 15, pages 176 to 208, 1968, incorporated herein by reference. Phenol itself is the phenol used in the greatest volume for the formation of such phenolic resins, but resorcinol, alkyl substituted phenols such as cresols, xylenols, and p-tert-butylphenol and p-phenylphenol are used in substantial volume. In the past, the aldehyde used has been almost exclusively formaldehyde, but small amounts of acetaldehyde and furfuraldehyde have also been used. The condensation of a phenol with an aldehyde is typically an acid catalyzed reaction, often an oxalic acid catalyzed reaction, with a molar ratio of aldehyde to phenol less than 1.
Early novolak resins used for photoresist manufacture have been modified for specific purposes. For example, prior to cure, novolaks of a phenol and formaldehyde have moderate thermal stability and typically melt within a range of from about 90.degree. C. to 120.degree. C., dependent upon the composition of the resin and its molecular weight. Recent developments have created novolak resins with increased melt temperature. For example, as disclosed in U.S. Pat. No. 4,424,315, an effort to increase thermal stability of novolak resins comprised acid condensation of a mixture of a naphtha and a phenol with an aldehyde. The resins so produced are copolymers formed by acid condensation of an aldehyde with an aromatic alcohol mixture of a naphtha and a phenol in the presence of an acid catalyst. These resins exhibit improved resistance to flow at elevated temperatures, though it was found that photoresists formulated with such resins were difficult to develop. In U.S. Pat. No. 4,943,511, a positive photoresist composition is disclosed which uses a resin binder that is prepared from a phenolic component having a high p-cresol content and an aldehyde that is a mixture of formaldehyde and an aromatic aldehyde. In accordance with the patent, photoresists formulated using the resin possess improved resolution capabilities, but it is believed that the resin exhibits only minimal thermal improvement. U.S. Pat. No. 5,216,111 is directed to resins comprising the condensation product of a phenol and an aromatic aldehyde and mixtures of such resins with other phenolic resins including conventional novolak resins. The resins disclosed in the patent exhibit glass transition temperatures in excess of 125.degree. C. and many exhibit glass transition temperatures as high as 175.degree. C. or higher. An alternative approach to the formation of aromatic novolak resins is disclosed in U.S. Pat. No. 5,238,776 where the resin is the product resulting from the acid condensation of a bishydroxymethyl phenol with another phenol in the absence of an aldehyde. By the process of this patent, high molecular weight resins are formed having excellent thermal stability.
In addition to modification of the structure of a phenolic resin as described above, it is also known that the phenolic resins may be modified by substitution of various groups onto pendant phenolic hydroxyl groups of the resin. For example, U.S. Pat. Nos. 4,968,581; 4,883,740; 4,810,613; 4,491,628 and 5,362,600 disclose substitution of certain acid labile blocking groups on the resin by esterification of an acid halide with the pendant hydroxyl groups. Exposure of a photoresist having a pendant acid labile group in a procedure that generates an acid cleaves the acid labile blocking group whereby exposure is reported to create areas of different solubility characteristics between exposed and unexposed areas of the polymer. Upon selective cleavage of the blocking group through exposure of the photoresist, a polar functional group is said to be provided, for example, carboxyl or imide.
An alternative reason for reaction of a phenolic resin with an acid halide is to modulate solubility or developability of an exposed resist coating. In this procedure, inert groups are condensed onto the phenolic hydroxyl group to decrease the solubility of the resin in a developer. Alkyl sulfonates and acid halides have been used for this purpose. In U.S. Pat. No. 5,541,263, phenolic resin having pendant phenolic hydroxyl groups are reacted with an acid halide to form an inert blocking group. By reaction of a portion, but not all of the hydroxyl groups, the solubility of a photoresist formulated with the blocked phenolic resin is decreased. It was reported that this decrease reduced or prevented microbridging during development.
A further modification of phenolic resins comprises reaction of pendant phenolic groups of the resin with radiation sensitive naphthoquinone diazide sulfonyl halides. In U.S. Pat. No. 5,271,918, a photoresist composition is disclosed wherein a relatively low molecular weight novolak resin is esterified with an o-quinone diazide sulfonyl halide. From 40 to 96 percent of the phenolic hydroxyl groups are condensed with the o-quinone diazide sulfonyl group. In U.S. Pat. No. 5,529,880, there is disclosed a photoresist composition comprising a novolak resin having phenolic hydroxyl groups condensed with an o-quinone diazide sulfonyl halide and an additional photoactive component that is the esterification product of an o-quinone diazide sulfonyl halide with a high molecular weight phenol having from 2 to 5 aryl groups and at least 4 hydroxyl groups with at least 50 percent of the phenolic hydroxyl groups condensed with the o-quinone diazide sulfonyl group.
Each of the patents discussed above is incorporated herein by reference for disclosure of novolak resin compositions, modifications of novolak resins and the use of such resins in photoresist compositions. For purposes of the invention described herein, the term "phenolic resin" means any phenolic resin used in the manufacture of photoresist inclusive of those phenolic resins formed from phenol and those phenolic resins modified in any way such as in accordance with the patents described above.
In processes for the manufacture of phenolic resins having pendant hydroxyl group esterified with an acid halide, such as described above, the monomers are reacted together to form the resin, the resin is typically precipitated in water and removed from the reaction mixture by filtration and then condensed with the acid halide in the presence of a base catalyst. The process results in the formation of salts formed from acid residues and the base catalyst. It is known that both the acid condensation of the components to form the phenolic resin and the further reaction to condense pendant phenolic hydroxyl groups with acid halides results in a phenolic resin containing high levels of acid salts formed by reaction with the base catalyst. It is further known in the art that such acid salts are unacceptable contaminants in photoresists used for high resolution imaging such as in the fabrication of integrated circuit devices.
Efforts to remove dissolved contaminants from materials used for the formation of photoresists by water washing and ion exchange are known in the art. For example, one such method is disclosed in International Publication No. WO 93/12152 directed to removing metal ions such as sodium and iron from novolak resins during their manufacture. The process comprises cation exchange treatment whereby a cation exchange resin is first washed with a mineral acid solution to reduce the level of total sodium and iron ions in the exchange resin to preferably less than 100 ppb, passing a formaldehyde reactant through the so treated cation exchange resin to decrease the sodium and iron ion content to less than 40 ppb, passing a phenolic compound through the cation exchange resin to decrease its sodium and iron ion content to less than 30 ppb, and then condensing the so treated phenolic compound with formaldehyde in the presence of an acid catalyst to form the resin. Other processes for removing contaminants from photoresist components are shown in U.S. Pat. Nos. 5,443,736; 5,472,616; 5,500,127; 5,518,628; 5,525,315; and 5,571,657. Each cited patent is incorporated herein by reference for disclosure of ion exchange procedures.
Though washing with water and ion exchange have been used to remove contaminants from photoresist materials in the prior art, it has been found that conventional procedures such as those described above are unsatisfactory for removing contaminants from phenolic resins containing acid salts of a base catalyst. Without wishing to be found by theory, it is believed that the higher molecular weight resins entrap the salts formed by the condensation reaction and prevent their removal by filtration, water washing or conventional ion exchange procedures.